Normal "Dry Season" favoured for Northern Australia

The north Australian outlook for total rainfall over the middle part of the
"Dry Season" (June to August), suggests a weak to moderate shift in the odds
favouring a drier than normal season over the Northern Territory. However, the
northern half of the NT and much of northern Queensland are seasonally dry
during winter in any case, with heavy falls a rarity.

The pattern of seasonal rainfall odds across northern Australia is due to recent
higher than average temperatures in both the Pacific and Indian Oceans. The
influences from these two oceans counteract each other in southern Queensland:
the Indian Ocean pattern promotes wetter conditions while the Pacific biases the
climate towards a drier than normal season.

The chances of exceeding the median rainfall for the July to August period are
below 40% over much of the Northern Territory. This means that for every ten years
with ocean patterns like the current, about six June-August periods are expected
to be drier than average over this broad region of the Northern Territory,
while about four periods are wetter.

Across the rest of the Northern Territory and Queensland, the chances of
exceeding the median winter rainfall are between 40 and 60%, meaning that
above average falls are about as equally likely as below average falls.

An expanded set of seasonal rainfall outlook maps and tables, including the probabilities of seasonal rainfall
exceeding given totals (e.g. 200 mm), is available on the "Water and the Land" (WATL)
part of the Bureau's website.

Outlook confidence is related to how consistently the Pacific and Indian
Oceans affect north Australian rainfall. During the July to August period,
history shows this effect to be moderately consistent across much of
Queensland and the Northern Territory (see background information).

The 2009/10 El Niño event across the Pacific Basin has now concluded.
Most leading climate models indicate further cooling during the coming months,
with the possible development of La Niña conditions by late winter or spring.
The SOI is approximately +6 for the 30 days ending 22 May. For routine updates
and comprehensive discussion on any developments regarding
El Niño and La Niña, please see the ENSO Wrap-Up.

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More information on this outlook is available
by contacting the Bureau's
Climate Services sections in Queensland and the
Northern Territory at the following numbers:

Background Information

The Bureau's seasonal outlooks are general statements about the probability
or risk of wetter or drier than average weather over a three-month period.
The outlooks are based on the statistics of chance (the odds)
taken from Australian rainfall/temperatures
and sea surface temperature records for the tropical Pacific and Indian Oceans.
They are not, however, categorical predictions about future rainfall,
and they are not about rainfall within individual months
of the three-month outlook period.
The temperature outlooks are for the average maximum and minimum temperatures
for the entire three-month outlook period.
Information about whether individual days or weeks may be
unusually hot or cold, is unavailable.

Probability outlooks should not be used as if they were categorical forecasts.
More on probabilities is contained in the booklet
The Seasonal Climate Outlook - What it is and how to use it,
available from the National Climate Centre.
These outlooks should be used as a tool in risk management
and decision making.
The benefits accrue from long-term use, say over 10 years.
At any given time, the probabilities may seem inaccurate,
but taken over several years, the advantages of taking account
of the risks should outweigh the disadvantages.
For more information on the use of probabilities,
farmers could contact their local departments of agriculture or primary industry.

Model Consistency and Outlook Confidence:
Strong consistency means that tests of the model on historical
data show a high correlation between the most likely
outlook category (above/below median)
and the verifying observation (above/below median). In this
situation relatively high confidence can be placed in the
outlook probabilities.
Low consistency means the historical relationship, and
therefore outlook confidence, is weak.
In the places and seasons where the outlooks are most skilful,
the category of the eventual outcome (above or below median)
is consistent with the category favoured in the outlook
about 75% of the time.
In the least skilful areas,
the outlooks perform no better than random chance or guessing.
The rainfall outlooks perform best in eastern and
northern Australia between July and January,
but are less useful in autumn and in the west of the continent.
The skill at predicting seasonal maximum temperature peaks in
early winter and drops off marginally during the second half of the year.
The lowest point in skill occurs in early autumn.
The skill at predicting seasonal minimum temperature peaks in
late autumn and again in mid-spring.
There are also two distinct periods when the skill is lowest
- namely late summer and mid-winter.
However, it must always be remembered that the outlooks
are statements of chance or risk.
For example, if you were told there was a 50:50 chance
of a horse winning a race but it ran second,
the original assessment of a 50:50 chance could still have been correct.

The Southern Oscillation Index (SOI) is calculated using the
barometric pressure difference between Tahiti and Darwin.
The SOI is one indicator of the stage of El Niño or La Niña
events in the tropical Pacific Ocean.
It is best considered in conjunction with sea-surface temperatures,
which form the basis of the outlooks.
A moderate to strongly negative SOI (persistently below −10) is
usually characteristic of El Niño, which is often associated with below
average rainfall over eastern Australia, and a weaker than normal monsoon
in the north. A moderate to strongly positive SOI (persistently above +10)
is usually characteristic of La Niña, which is often associated with
above average rainfall over parts of tropical and eastern Australia,
and an earlier than normal start to the northern monsoon season.
The Australian impacts of 25 El Niño events since 1900 are summarized
on the Bureau's web site
(El Niño - Detailed Australian Analysis).